The radio spectrum assigned to a mobile communications system must in general be reused within different geographical sub-areas, so called cells, in order to provide both coverage and capacity. Depending on technology choices and factors such as what multiple access techniques that is employed and how robust to interference the mobile and base station receivers are, the so called reuse distance between different cells using the same portion of spectrum may however vary. This variation appears, not only between different types of systems, but also within the same system. The latter is due to e.g. the topology of the service area.
In practice, to model the cellular layout of a mobile communications system a heterogeneous grid of regular hexagons is often used. Although not providing a true description of the real cell and its coverage area, as an approximation, hexagons have proven useful for cell planning purposes as they provide a convenient framework in which a wide range of tessellating cell-reuse clusters can be defined to describe the distribution of the available spectrum/channel resources over the total service area of a given system.
It is known that tessellating clusters of size N can be constructed ifN=i2+ij+j2,where i and j are non-negative integers and i≧j. From the relation above, it follows that the allowable cluster sizes are N=1, 3, 4, 7, 9, 12, . . .
Clearly, to optimise the spectrum efficiency of a given system, it is desirable to use as small cluster reuse factor as possible. However, unless spread spectrum or space division multiplexing techniques are used, employing small reuse factors may not be possible since the impact of co-channel interference from neighbouring cells may become too severe. That is, the impact of co-channel interference may potentially wreck the ability of the receivers in the different cells to demodulate and recover their intended data meeting quality of service expectations. To circumvent this problem and to gain control over the emission and impact of co-channel interference reuse factors larger than one are typically used in many systems in real life.
Employing a fixed reuse factor larger than one automatically implies that only a portion of the available system bandwidth is offered to any mobile station in a given cell. This is clearly, both from a network point-of-view as well as from a user perspective, non-advantageous for many reasons. A small selection of such reasons are:                The maximum peak throughput rate is reduced.        The mobile station exposure to co-channel interference increases as the transmission time increases. Hence, the probability that base stations in nearby cells simultaneously transmits packets to different users using the same channel resources increases.        Base station emission period of interference into neighbouring cells increases.        The accessibility of the channel resource is reduced. Serving many mobile stations, the delay in the base station may become substantial. This will be due to both the actual transmission time but also to the time required for retransmissions.        Linked to the reduced peak data rate and the deteriorated channel accessibility is also the risk that the user perception of the air-interface as being slow may increase.        
One example of frequency planning is disclosed in the U.S. Pat. No. 6,498,934. Enhanced path-loss estimates are here used for assigning channels to different base stations. The path-loss estimates are obtained by instructing mobile stations being connected to the system to measure certain neighbouring cell signals and to lock the mobile stations power to enable synchronized measurements in neighbouring base stations. From these measurements, statistics on path-loss estimates are calculated, which in turn are used for improving the frequency planning.
In the published US patent application 2003/0013451 A1, a method is disclosed, where the reuse plan for the cells of a communications system is dynamically redefined. Based on a number of factors, such as the observed interference levels, loading conditions, system requirements etc, the reuse plan for the division of resources to the different cells can be adapted. The publication also discloses methods for efficient allocation of resources within the available set of resources for each cell.
A problem with the reuse plan adaptation presented in US 2003/0013451 A1 is that the entire communications system has to involved in the adaptation. Resources that are influenced by the adaptation have to be unused and system configuration data has to be updated throughout the entire system before the new reuse plan can be utilised. This problem makes it less advantageous to use the adaptation ideas, at least for adapting to short-term changes in the communications system.